1. Field of the Invention
The present invention relates to a scanning optical system and an image forming apparatus using the same. Specifically, the present invention relates to a scanning optical system that is adapted to reflect and deflect a light flux emitted from a light source by means of a polygon mirror serving as a light deflector and have an fxcex8 characteristic, which is preferable for use in an image forming apparatus, such as a laser beam printer including an electrophotography process, a digital copying machine or a multi-function printer, which records image information by optically scanning a surface to be scanned through scanning optical means including an optical element provided with a fine structure grating.
2. Related Background Art
In conventional scanning optical systems of laser beam printers etc., a light flux emitted from light source means that has been modulated in accordance with an image signal is cyclically deflected by a deflector composed, for example, of a polygon mirror and converged into a spot on a surface of a photosensitive recording medium by scanning optical means having an fxcex8 characteristic, so that an image is recorded.
FIG. 16 is a cross sectional view taken in the main scanning direction (main scanning cross sectional view) showing the principal portion of a conventional scanning optical system (i.e. optical scanning apparatus).
In FIG. 16, reference numeral 91 designates light source means, which is composed, for example, of a semiconductor laser or the like. Reference numeral 92 designates a collimator lens, which converts a divergent light flux emitted by the light source means into a parallel light flux. Reference numeral 93 designates an aperture stop, which restricts a light flux passing through it to shape the beam. Reference numeral 94 designates a cylindrical lens, which has a certain power only in the sub-scanning direction, to image the light flux having passed through the aperture stop 93 onto a deflection surface (or reflection surface) 95a of a light deflector 95 (which will be described below) as a substantially linear image in a cross section in the sub-scanning direction.
Reference numeral 95 designates a light deflector serving as deflecting means, which is composed, for example, of a polygon mirror (i.e. a rotatory multi-surface mirror) having four faces. The light deflector 95 is rotated in the direction indicated by arrow A in FIG. 16 at a constant speed by driving means (not shown) such as a motor etc.
Reference numeral 96 designates a scanning lens system serving as scanning optical means having a light collecting function and an fxcex8 characteristic, which is composed of first and second scanning lenses (two lenses in all) 96a and 96b. The scanning lens system 96 images a light flux corresponding to image information that has been reflected and deflected by the light deflector 95 onto a surface to be scanned, that is, a surface 97 of a photosensitive drum, while realizing a conjugate relationship between the deflection surface 95a of the light deflector 95 and the photosensitive drum surface 97 in the sub-scanning cross section, to perform a field tilt correcting function in order to correct the surface inclination of the deflection surface 95a. 
In FIG. 16, a divergent light flux emitted from the semiconductor laser 91 is converted by the collimator lens 92 into a substantially parallel light flux, and then the light flux is restricted (in light quantity) by the aperture stop 93 and incident on the cylindrical lens 94. The substantially parallel light flux incident on the cylindrical lens 94 emerges from it without any modification with respect to the main scanning cross section. On the other hand, in the sub-scanning cross section, the light flux is converged so as to be imaged onto the reflecting surface 95a of the light deflector 95 as a substantially linear image (namely, a linear image that is longitudinal in the main scanning direction). The light flux reflected and deflected by the reflecting surface 95a of the light deflector 95 is imaged by means of the first and second scanning lenses 96a and 96b onto the photosensitive drum surface 97 as a spot, whereby the imaged light spot scans the photosensitive drum surface 97 in the direction indicated by arrow B (i.e. the main scanning direction) at a constant speed, as the light deflector 95 is rotated in the direction of arrow A. Thus, an image is recorded on the surface 97 of the photosensitive drum as a recording medium.
However, the conventional scanning optical system as described above suffers from the problems as described in the following.
Recently, it has become a general practice to manufacture scanning optical means (i.e. scanning lens systems) of a scanning optical system using a plastic material, which is easy to process into an aspheric shape and with which the manufacturing is easy. However, it is difficult to apply anti-reflection coatings on plastic lenses for technical and economical reasons. So, plastic lenses suffer from Fresnel reflection generated at their optical surfaces.
FIG. 17 is a graph showing incident angle dependency of reflectance and transmittance of an example of an optical element made of a resin, having a refractive index n=1.524, under a condition in which P-polarized light is incident on that element. As will be seen from the graph, the surface reflection at each surface becomes large, as the angle of incidence increases.
In connection with this, since in the scanning optical means, the angle of incidence generally varies as the position of incidence changes away from the on-axis position toward an off-axis position, the Fresnel reflection at each optical surface also varies largely. As a result, there is a difference between the light quantity at the on-axis position and the light quantity at the off-axis position. As the angle of incidence increases from 0 degree to the Brewster""s angle, the reflectance decreases (i.e. the transmittance increases), and therefore the transmittance of the whole system increases as the position changes from the on-axis position to an off-axis position. Therefore, the illuminance distribution on the surface to be scanned also increases toward the off-axis position. It will be seen from FIG. 17 that the light quantity at the outermost off-axis position is larger than the light quantity at the on-axis position by 5%. As a result, a density difference would be created in an image output by the image forming apparatus between the central portion and the peripheral portion thereof, which is a problem.
As a solution for the above-mentioned problem, Japanese Patent Application Laid-Open No. 2000-206445 proposes adjusting the diffraction efficiency of a diffraction grating surface provided in scanning optical means appropriately in order to eliminate that problem. Specifically, it proposes adjusting the depths of cuts of diffraction grating surface, on which grating is cut at a predetermined pitch that realizes a desired power distribution for the purpose of correcting chromatic aberration of magnification or correcting focus, to vary the diffraction efficiency of the diffracted light (i.e. the first-order diffracted light) so as to cancel the variation in transmittance created by other refracting surfaces.
However, the diffraction grating proposed by Japanese Patent Application Laid-Open No. 2000-206445 suffers from a problem as described below.
It is known that when the pitch of a grating becomes as small as or smaller than the wavelength of light (i.e. a fine structure grating), it shows a structural birefringence.
In xe2x80x9cKOUGAKU-NO-GENRI Vol. IIIxe2x80x9d (a Japanese translation of xe2x80x9cPrinciple of Opticsxe2x80x9d by Max Born and Emil Wolf), Tokai University Press, p1030, it is describes that regularly arranged particles each of which is made of optically isotropic material and having a size sufficiently larger than its molecule size and smaller than the wavelength of light show a structural birefringence. In other words, a model in the form of an aggregation of thin plane parallel plates having a periodicity equal to or smaller than the order of the wavelength as described in xe2x80x9cKOUGAKU-NO-GENRIxe2x80x9d becomes a kind of uniaxial crystal whose effective dielectric constant (or permeability), which is obtained based on the dielectric constant of the medium of the plane-plate portion and the dielectric constant of the medium of the non-plane-plate portion, behaves differently for an electric vector parallel to the plane-plates and an electric vector perpendicular to the plane-plates.
More specifically, the fine structure grating that has a grating pitch substantially equal to or smaller than the wavelength of light has different refractive indices with respect to the direction parallel to the direction of the arrangement of the grating and to the direction orthogonal to the direction of the arrangement of the grating, depending on the direction of the plane of polarization of an incident light flux.
Due to the above-described fact, it is impossible to obtain desired transmission and reflection characteristics without setting an appropriate grating pattern in accordance with the polarization of an incident light flux. Japanese Patent Application Laid-Open No. 2000-206445 (U.S. Pat. No. 6,222,661) does not teach this point sufficiently. Particularly, in Japanese Patent Application Laid-Open No. 2000-206445, it is assumed that a concentric grating is used, and so when a beam having a certain image height crosses the grating surface, the direction of grating within the light flux is not constant.
Furthermore, there is such a beam synthesizing system in which two laser light flux having different linear polarization are synthesized by a polarizing beam splitter, reflected and deflected by a light deflector, and then imaged in a scanning manner onto a surface to be scanned by an imaging optical element, which is disclosed, for example, in Japanese Patent Application Laid-Open No. 11-218699. In this type of beam synthesizing system, light fluxes having two polarization states are incident on an imaging optical system. If a fine structure grating having structural birefringence as described above is used in this type of scanning optical system, transmission and reflection characteristics vary depending on the polarization state. Consequently, a difference in light quantity on the image plane will be generated between a plurality of laser light fluxes, and it is impossible to realize uniform exposure, which is a problem.
An object of the present invention is to reduce influence of the structural birefringence of a fine structure grating provided on an optical surface in a scanning optical system to provide a scanning optical system having good optical properties that is not depend on the polarization state of an incident light flux and to provide an image forming apparatus using the same.
It is another object of the present invention to provide a scanning optical system that can reduce Fresnel (surface) reflection at a lens surface that would cause flare and ghost images and to provide an image forming apparatus using the same.
According to the present invention, there is provided a scanning optical system in which a light flux emitted from a laser light source is deflected by deflecting means and the light flux having been deflected by the deflecting means is imaged by scanning optical means onto a surface to be scanned so as to scan the surface to be scanned, wherein:
the scanning optical means comprises at least one optical surface that has a fine structure grating having a grating pitch smaller than the wavelength of the light flux emitted from said laser light source; and
the direction of arrangement of the fine structure grating is the same all over the surface of the fine structure grating.
In this scanning optical system according to the invention, it is preferable that the direction of arrangement of said fine structure grating and the direction of a plane of polarization of an incident light flux have a constant relationship.
In the scanning optical system according to the present invention, the direction of arrangement of said fine structure grating may be either one of a direction parallel to a plane of polarization of an incident light flux, a direction perpendicular to the plane of polarization of the incident light flux or a direction that forms an angle of 45 degrees with the plane of polarization of the incident light flux, or including at least two of these directions.
In the scanning optical system according to the present invention, the direction of arrangement of said fine structure grating may be either one of a direction parallel to a main scanning plane, a direction perpendicular to the main scanning plane or a direction that forms an angle of 45 degrees with the main scanning plane, or including at least two of these directions.
According the invention there is also provided a scanning optical system in which a plurality of light fluxes emitted from a plurality of laser light sources are deflected by deflecting means and the light fluxes having been deflected by the deflecting means are imaged by scanning optical means onto a surface to be scanned so as to scan the surface to be scanned, wherein:
the scanning optical means comprises at least one optical surface that has a fine structure grating having a grating pitch smaller than the wavelength of the light flux emitted from the laser light source; and
the directions of polarization of a plurality of light fluxes incident on the fine structure grating are different from each other, and the direction of arrangement of the fine structure grating is arranged to be line symmetrical with respect to composition of vectors representing the directions of polarization of said plurality of light fluxes.
In this scanning optical system according to the invention, the plurality of light fluxes incident on the fine structure grating may be P-polarized light and S-polarized light with respect to a incidence surface of the fine structure grating, and the direction of arrangement of the fine structure grating may be arranged to be line symmetrical with respect to a direction that forms an angle of 45 degree with the incidence surface of the fine structure grating.
According to the present invention there is also provided an image forming apparatus comprising:
a scanning optical system as described in the foregoing;
a photosensitive member disposed at the surface to be scanned;
a developing device that develops an electrostatic latent image formed on the photosensitive member by a scanning light flux from the scanning optical system as a toner image;
a transferring device that transfers the developed toner image onto a transferring material;
a fixing device that fixes the transferred toner image on the transferring material;
a printer controller that converts code data input from an external device into an image signal and input it to the scanning optical system.